Which is why the most fundamental role of the Armed Forces is not to fight wars, but – through their very existence – to deter, and thus to prevent war.

For no part of our Armed Forces is that more true than our nuclear capability. If nuclear weapons are fired, they have failed. But they are used every day: to deter.

This Government was elected on a manifesto that included a commitment to build four new ballistic missile submarines … replacing the Vanguard submarines that come out of service from the early 2030s.

And we’ve committed to a debate and vote this year so that Parliament can endorse that decision. So now is the right time to set out why we should retain our nuclear deterrent.

There are three reasons.

Because we are realistic about the world we live in.

Because we take our responsibilities to the British people and to our Allies seriously.

And because that means that nuclear weapons are relevant now and are going to be relevant for the foreseeable future.

Let me take each in turn.

1. REALISM

First, it’s about realism.

Some characterise this debate as one of extremes. Between those who want to disarm and those who never will disarm.

Let me reject that artificiality. We all agree on the destructive power of nuclear weapons, and that we must do everything to ensure they will never be used.

We also have a shared ambition to see a world where nuclear weapons states feel able to relinquish them.

Where we really differ is how best to achieve this.

On the one hand are those idealists who believe that unilateral disarmament will make us safer…

…on the other are those of us who recognise that the real world threats to the United Kingdom are growing not diminishing.

So we must be realistic about the world in which we live.

The Labour Government’s 2006 White Paper on the future of the deterrent identified risks to the UK from major nuclear armed states from emerging nuclear states, and state sponsored terrorism.

Those risks have not gone away.

Indeed, nine years on, our own SDSR judged that the United Kingdom is facing challenges that are growing in scale, diversity, complexity and in concurrency.

Nor has the nuclear threat gone away. The SDSR recognised, and I quote, “continuing risk of further proliferation of nuclear weapons” and concluding that we could not “relax our guard… or rule out further shifts which would put us under grave threat”.

And Russian behaviour is a case in point here.

Russia has become more aggressive, more authoritarian and more nationalist. Its illegal annexation of Crimea and support of Ukrainian separatists through the use of deniable, hybrid tactics and media manipulation have shown its willingness to undermine the rules based international system in order to promote and secure its own perceived interests.

Russia is upgrading its nuclear forces; and Russia is threatening to base nuclear forces in Kaliningrad and in the Crimea.

The last two years have seen a worrying increase in both official Russian rhetoric about the use of nuclear weapons and the frequency of snap nuclear exercises.

And we should take heed of those developments.

North Korea is another worrying case study. North Korea is the only nation to have tested nuclear weapons in the 21st century. It now claims to have withdrawn from the Nuclear Non-Proliferation Treaty. It’s developing long-range missiles, and continues to flaunt its new found nuclear capabilities.

Just as we must be realistic about the growing nuclear threats, we also have to acknowledge that our prospects of single-handedly convincing the world to abandon its nuclear arms… are limited.

Now we are committed to creating the conditions for a world without nuclear weapons, in line with our obligations under the Nuclear Non-Proliferation Treaty.

And we have led the way on disarmament.

We’ve cut our nuclear stockpiles by over half since the height of the Cold War

Last year I reduced the number of deployed warheads on each submarine from 48 to 40 last year

And we have pledged to reduce further our stockpile of nuclear weapons to no more than 180 warheads by the mid-2020s.

Other nations have not followed suit.

There remain about 17,000 nuclear weapons in the world… we have less than one per cent of them.

It is frankly naïve – even vainglorious – to imagine that the grand gesture of UK unilateral disarmament could change the calculations of nuclear states, or those seeking to acquire weapons.

Far more likely they would see it as weakness.

So the only way to create the global security conditions necessary for achieving nuclear disarmament is by working multilaterally…

by taking tangible steps towards a safer and more stable world

And by giving states with nuclear weapons the confidence they require to relinquish them.

Our recent efforts, working alongside other leading powers, secured a deal with Iran and showed what can be achieved.

But we should also be realistic about how long this will take.

As the great nuclear theorist and former MOD Permanent Secretary, Sir Michael Quinlan, once wrote:

‘no safer system than deterrence is yet in view, and impatience would be a catastrophic guide in the search. To tear down” he said… “the present structure, imperfect but effective, before a better one is firmly within our grasp would be an immensely dangerous and irresponsible act.’

2. RESPONSIBILITY

That brings me to my second point. We have a political and moral responsibility to our people and our Allies.

No-one would claim the nuclear deterrent solves all of our national security requirements.

Terrorist threats are all too real – as we saw so tragically yesterday. But nuclear weapons were never intended to combat terrorism.

They are intended to deter the most extreme dangers our nation might face.

NATO is the cornerstone of our defence. It is first and foremost a defensive alliance. And it is also a nuclear alliance.

By maintaining our independent nuclear deterrent, alongside the US and France, we provide NATO with three separate centres of decision-making.

That complicates the calculations of potential adversaries, and it prevents them threatening the UK, or our allies, with impunity.

Now some will ask why we possess nuclear weapons when other Allies such as Germany do without them.

But we can’t rewrite history. We were one of the original nuclear powers. Others were not.

And many of those Allies signed the Non-Proliferation Treaty in the late 1960s in the knowledge they were covered by NATO’s nuclear umbrella, including the United Kingdom deterrent.

It would not be the action of a strong and valued ally to withdraw that protection.

And how would the United States or France respond if we suddenly announced that we were abandoning our nuclear capabilities…

…yet will still expect them to pick up the tab and to put their cities at risk to protect us in a nuclear crisis?

Without our nuclear contribution to NATO, could we guarantee that a potential adversary might not miscalculate the degree of United States commitment to the defence of Europe?

As one of the leading member of NATO we shouldn’t now think of outsourcing our commitments.

That would not make us safer and it would have no moral merit.

It would weaken us now and in the future.

It would undermine NATO.

And it would embolden our adversaries.

3. RELEVANCE

That brings me to my third point: the relevance.

Our independent nuclear deterrent is relevant not only for today but also for the foreseeable future.

The UK case does not rest on our assessment of threats that face us now….

…but on our assessment of what the world could be like in the 2030s, 2040s, 2050s and beyond…

And the truth is we don’t know.

No-one accurately predicted the end of the Cold War…or the coming of the Arab Spring, Russia’s annexation of Crimea, or the rise of Daesh.

Those who argue in favour of scrapping our nuclear deterrent unilaterally must be certain - absolutely certain - no extreme threats will emerge in the next 30 or 40 years to threaten our security and way of life.

And they can’t be so certain.

That is why successive Governments for over sixty years have concluded that this country should retain its nuclear weapons.

Now the UK government last formally presented the case for the future of the UK’s Nuclear Deterrent to Parliament in 2006.

Launching that White Paper Tony Blair, said “an independent nuclear deterrent is an essential part of our insurance against the uncertainties and risks of the future.”

That was the right judgment then.

It’s the right judgment now.

Our nuclear deterrent has helped keep the peace between the major powers for decades.

Abandoning it, would undermine our security and that of our allies. It would not make us safer.

And once we gave up those weapons, there would be no going back to them.

OBJECTIONS

That is the case for retaining a nuclear deterrent.

And I put it to you that it is hard to argue against the principle.

But, before concluding, let me finally address the main practical objections that people have raised.

First, the claim that there are cheaper and more effective ways of providing a similar effect to the Trident system.

There aren’t.

Successive studies have looked at this in detail…

… under Labour Ministers in 2006…

…and more recently under the Conservative and Liberal Democrat Coalition in 2013.

The Trident Alternatives Review in 2013 assessed what ships, aircraft, submarines and silos could deliver nuclear weapons; and which missiles, bombs or nuclear warheads were most appropriate.

It found that submarines were less vulnerable to attack than silos or aircraft.

They can maintain a continuous posture in a way that aircraft and land-based alternatives cannot.

It made clear that alternative delivery systems, such as cruise missiles, wouldn’t have the same range as the Trident missile … reducing the reach and capability of our deterrent.

Only the current submarine-based, Trident missile system offered the resilience but also the cost-effectiveness that successive UK Governments sought. The second objection is that submarines will somehow become obsolete, through technological developments such as unmanned underwater vehicles or cyber threats.

The ocean is a vast, complex and challenging environment in which to conduct large scale anti-submarine warfare.

Our confidence that submarines will not be rendered obsolete by technology is partly based on classified analysis, but also on some obvious facts.

Operating quietly for long periods in the ocean is highly demanding. It requires endurance, a powerful energy source, resilience from high pressure and corrosion, and stealth.

The ability to track submarines and then communicate their position brings with it many significant challenges.

Now we dedicate considerable resource to assessing these emerging capabilities. And we judge that there is no inherent reason, for the foreseeable future, to believe that unmanned submarines will be substantially more difficult to counter, than manned submarines.

As for cyber-attack, while deployed, submarines operate in isolation. It’s hard to think of a system less susceptible to cyber-attack.

And it’s also worth asking, if nuclear submarines were redundant, or going to be redundant, why would the United States, China, Russia and France all be spending tens of billions of dollars on their own submarine based ballistic missile systems?

As practical as these objections appear, they are in fact simply the latest in a litany of arguments employed to justify an anti-nuclear position.

COST

The third practical objection is cost.

Now we must remember that security underpins all the Government’s priorities.

With the fifth biggest defence budget in the world…backed up by our commitment to invest 2 per cent of GDP in defence… we can afford conventional and nuclear capabilities.

Our estimate is that four new submarines will cost £31 billion to build. We’ve also set a contingency of £10 billion on top of that.

But the £31 billion acquisition cost will be spread over 35 years, which works out as an insurance premium of 0.2% per year of total Government spending.

Twenty pence in every £100 pounds the Government spends…for a system that will provide a capability through to the 2050s and beyond.

I believe that that is a price worth paying.

CONCLUSION

So let me say in conclusion…before nuclear weapons, major powers embarked on two of the most destructive wars imaginable.

Many millions died, millions more suffered.

Yet, for all the conventional conflicts since, and there have been many of them, there hasn’t been major conflict between nuclear armed states. The devastating possibilities of nuclear war have helped maintain strategic stability.

And our independent UK nuclear deterrent has played its part.

Those who still oppose it must prove to us how relinquishing it would make us safer.

Now we should not accept nuclear deterrence as the last word in ensuring freedom from major war. Our commitment under our Non-Proliferation Treaty obligations is clear.

But to abandon our deterrent now would be an act of supreme irresponsibility.

In 2007 Parliament voted to maintain the minimum strategic nuclear deterrent beyond the life of the existing system.

Last year Parliament voted twice to retain our deterrent.

This year Parliament will have the opportunity to vote on the principle of Continuous At Sea Deterrence and our plans for Successor.

This is not a judgment about short term threats.

It is about the threats we may face over generations to come.

We should not gamble with our national security.

The United Kingdom’s independent nuclear deterrent remains right for our nation – for as long as the global security situation demands.